In the manufacture of certain wet-laid paper products, such as facial tissue, bathroom tissue, or paper towels, the web is conventionally subjected to a creping process in order to give the web desirable textural characteristics, such as softness, bulk, stretch, and absorbency. The creping process involves the adhering of the web to a rotating creping cylinder, such as an apparatus known as a Yankee dryer, and then dislodging the adhered web from the cylinder with a doctor blade. The impact of the web against the doctor blade ruptures some of the fiber-to-fiber bonds within the web and causes the web to wrinkle or pucker.
The severity of this creping action is dependent upon several factors, one of which is the degree of adhesion between the web and the surface of the creping cylinder. Greater adhesion causes increased softness, although generally with some loss of strength. In order to increase adhesion, a creping adhesive is generally sprayed onto the surface of the creping cylinder to enhance any naturally occurring adhesion that the web may have due to its water content when applied to the creping cylinder. The water content of the web will vary widely depending on the extent to which the web had been previously dried.
Generally, creping aids are comprised of an adhesive component and a release component. Creping aids should prevent wear of the dryer surface, provide lubrication between the doctor blade and the dryer surface, and reduce chemical corrosion, as well as control the extent of creping.
A wide variety of creping adhesives are known in the art. Examples of creping adhesives are polyvinyl alcohol, ethylene/vinyl acetate copolymer, animal glue, polyamidoamine-epichlorohydrin resins (PAE resins) and polyvinyl acetate.
U.S. Pat. No. 4,528,316 discloses a creping adhesive comprising an aqueous admixture of polyvinyl alcohol and a water-soluble, thermosetting, cationic polyamide resin which provides increased adhesion in the manufacture of creped wadding.
U.S. Pat. No. 5,338,807 discloses a creping aid comprising the reaction product of a polyamide of a dibasic acid or of the ester of an aliphatic dibasic acid and methyl bis (3-aminopropylamine) with epichlorohydrin in a mole ratio of the polyamide to the epichlorohydrin between about 1:0.1 and about 1:0.33.
U.S. Pat. No. 5,382,323 discloses an improved halogen-free adhesive obtained by reacting adipic acid with diethylenetriamine at equimolar ratios of from 1.2:1.0 to 1.0:1.2 and then crosslinking with a dialdehyde selected from gluteraldehyde, glyoxal, or mixtures thereof. Improved adhesion and full strength are obtained.
U.S. Pat. No. 5,944,954 discloses a creping adhesive comprising cationic starch and optionally a polyvinyl alcohol and a water-soluble thermosetting cationic polyamide-epihalohydrin resin which provides high adhesion and doctorability for dry creping.
The amount of adhesion between the web and the metal surface of the creping cylinder of the Yankee dryer at the creping or doctor blade plays a significant role in the development of the properties of the tissue and/or towel product. Soerens, in U.S. Pat. No. 4,501,640 has demonstrated that increased adhesion provided by creping adhesives (Yankee dryer adhesives) gives increased softness to the manufactured tissue. Therefore, the ability to provide increased adhesion of the wet paper sheet or web to the Yankee dryer surface is a desirable attribute of a creping adhesive.
In addition to providing a desirable amount of adhesion, it is also desirable to prevent wear of the Yankee dryer surface. Thus, the creping adhesive must provide a coating that is durable enough to withstand the mechanical forces at the doctor blade. In the past, this was commonly done using a resin that crosslinked to a great extent onto the Yankee dryer surface. These resins were typically polyamidoamine-epichlorohydrin polymers that are referred to as PAE resins. Some such PAE resins are sold under the trade names Kymene.RTM., Rezosol.RTM., Cascamid.RTM., and Amrez.RTM.. These PAE resins are available respectively from the Hercules Chemical Company, the Houghton Company, the Borden Company, and Georgia Pacific. Generally, these resins are sprayed onto the Yankee dryer; however, they may also be added to the pulp slurry at the wet end of the paper machine.
Some of these commercially available PAE resins are wet-strength resins that contain enough epichlorohydrin to crosslink substantially on the Yankee dryer surface. These resins provide a durable coating on the dryer surface and a low amount of adhesion of the web to the dryer. However, after a period of time, the coating can become hard and can build up unevenly to the extent that the coating causes poor creping. Thus, the coating must be removed through the use of an abrasive.
The level of adhesion of the papermaking web to the Yankee dryer is important as it relates to the control of the web in its travel in the space between the creping or doctor blade and the winder upon which a roll of paper is being formed. Webs that are insufficiently adhered tend to cause poor control of the sheet with consequent difficulties in forming a uniform reel of paper. A loose sheet between the creping or doctor blade and the reel generally will result in wrinkles, foldovers, or weaving of the edges of the sheet in the rolled-up paper. Poorly formed rolls not only affect the reliability of the papermaking operation, but also tend to affect subsequent operations wherein the rolls are converted into a tissue or a towel product. The level of adhesion of the papermaking web to the Yankee dryer is also of vital importance as it relates to the drying of the web. Higher levels of adhesion improve heat transfer, thus causing the web to dry faster resulting in a more energy efficient, higher speed operation of the creping process.
Other commercially available resins are similar to the wet-strength PAE resins discussed hereinabove but have a much lower amount of crosslinking agent. This type of resin generally yields a coating that has moderate adhesion. However, the water resistance may tend to be "too" low due to the water solubility that results from having a low amount of crosslinking agent.
It is recognized in the industry that there is a need for an adhesive creping aid that provides a coating on the Yankee dryer that maintains a desired level of adhesion and water resistance while yielding a sheet of creped paper with the desired balance of properties.